1. Field of the Invention
This invention is directed to circuitry for producing and/or previewing transitions between video signals comprising a television program or the like.
2. Discussion of the Prior Art
As illustrated in FIG. 1, circuitry is known for previewing the next event of a sequence of events comprising a television program. A program transition switch 10 has applied thereto video signals A and B and a control signal C. Switch 10 may be a soft switch, the operation of which is such that the output signal is [A.times.C]+[(1-C).times.B]. Hence, if C is 0, only the video B signal is passed by the switch and if C is 1, only the video A signal. For values of C between 0 and 1, a mixture of the two signals occurs. The frequency range for the control signal may be anywhere from DC to the highest frequency in the video spectrum, typically 5MHz. Thus, a single soft switch is capable of performing a dissolve between two video signals (C frequency-DC) or acting as an instantaneous switch (C frequency-very high) or any combination of the two by varying only the control signal. A wideband analog multiplier readily performs these functions.
Assuming the "on-air" or program signal is video signal A, C will be 0 and video signal A will occur at the output of switch 10. The next event corresponds to B. In order to preview the next event signal B, signals A and B are also applied to a 2.times.1 switch indicated at 12 where the switch comprises cross-points 14 and 16. Hence, cross-point 16 can be actuated to permit preview of the next event on a monitor (not shown) connected to the output of switch 12.
In order to make a transition to the next event, the control signal C is changed from 1 to 0, the nature of the change determining the nature of the transition as indicated above. Assuming there is a dissolve to the next event, C would comprise a negative-going ramp signal. Once the transition has been completed, video signal B becomes the "on-air" or program event and the next or preview event is now on the A side. Signal A may now be composed in accordance with known circuitry to include, for example, a background signal B1, a foreground signal F chroma keyed over the background signal B1 and a title over the foreground and/or background signals F and B1. The program signal B now on-air may comprise a simple background scene B2. Once the next scene (video A) has been composed, the operator can preview it by actuating cross-point switch 14 where the P/V video output is connected to the monitor.
There is a shortcoming in the foregoing system in that the operator can only preview the next event and not the transition to that event. Since video signals A and B can become quite complicated as indicated above, the operator desirably not only needs to know what the next event will look like but also the transition to that event. Thus, assuming program or "on-air" event B is B2 and preview or next event A is background B1 with foreground F chroma keyed thereover and with a title over B1 and F and further assuming a wipe transition is desired from video signal B to A, it is desirable that the operator have the capability of previewing this transition before actually producing it on the "on-air" or program line. Further, it is desirable that this capability be readily available to the operator so that the transition preview can be simply effected.